It is well-known that ribonic acid and its salts are very important as intermediates for the synthesis of riboflavin (vitamin B.sub.2). The ribonates are obtained by oxidizing glucose, in aqueous sodium hydroxide solution or potassium hydroxide solution, by means of oxygen to give a mixture of Na arabonate and Na formate, or K arabonate and K formate, isolating the arabonate from this solution by adding a water-miscible solvent and then epimerizing the arabonate by heating in aqueous solution at 130.degree.-140.degree. C.
Since this epimerization always only proceeds to an equilibrium of about 70% of arabonate and 30% of ribonate, fractional crystallization of arabonate and ribonate is necessary. However, the sodium salts and potassium salts are not suitable for this purpose, since they are extremely water-soluble and furthermore do not differ sufficiently in their solubility. For this reason, the aqueous solutions containing Na or K arabonate and Na or K ribonate are in general mixed with calcium chloride, after which calcium arabonate preferably crystallizes out on cooling (cf. Japanese Published Patent No. 4225/1955). A variant of this method is first to convert the Na arabonate or K arabonate into Ca arabonate, isolate the latter and then carry out the epimerization (cf. U.S. Pat. No. 2,438,882). On fractional crystallization of the said epimeric calcium salts, a ribonate which is about 80% pure can be obtained.
To achieve greater purity, the Ca salts are converted to other salts, which can be purified by further crystallization. Separation methods employing the zinc, calcium and mercury salts have been disclosed, but these are unsuitable for an industrial process because of the toxicity of the metal salts and the resulting problem of disposing of the effluent.
It is true that the iron salts, also used for this purpose, are non-toxic, but they have other important disadvantages. For example, conversion of the calcium salts to the iron salts requires heating for from 3 to 5 hours at 80.degree.-100.degree. C., which is industrially unattractive. Furthermore, conversion of the iron ribonate into ribonolactone gives yields of only about 80-85%, which is unsatisfactory for an industrial method of synthesis.